1. Field of the Invention
The invention relates generally to test apparatus and to methods for the investigation of bearings or of their lubricants and more particularly concerns versatile test equipment and methods for the use thereof permitting testing of ball bearings and lubricants therefor under substantially the actual conditions in which they are expected to be used in commercial products such as gyroscopes. Furthermore, the apparatus and method are adaptable for performing fundamental research upon ball bearing configurations, including ball cages, or lubrication techniques and the lubricants themselves.
2. Description of the Prior Art
In the prior art, it has long been recognized that the proper generation and maintenance of the lubricant film between the bearing balls and races of an operating ball bearing assembly are critical with respect to bearing life and maintainability. This criticality is particularly present with bearings operating at high speeds or at relatively high temperatures. As a consequence, there have been many attempts in the prior art to measure elastohydrodynamic lubricant film thickness in actual bearings or in apparatus in which the bearing is simulated. Some of these measurement schemes have involved the electrical measurement of resistance, of dielectric breakdown voltage, or of the effective capacitance of the lubricant film under operating conditions. In addition, x-ray diffraction techniques and optical interferometric methods have been applied with certain degrees of success.
Because of their nature, most of the prior art test arrangements have involved specially prepared and instrumented bearing assemblies which usually merely simulate the actual bearing interfaces and thus do not give a reliably true picture of the behavior of an actual bearing lubricated with a particular lubricant in a situation which closely resembles the behavior of the film and bearing elements in a normal operating circumstance. On such example of a test device involving bearing simulation is taught in the T. B. Howe U.S. Pat. 3,178,928, issued Apr. 20, 1965, and assigned to Sperry Rand Corporation. In the Howe apparatus, important characteristics of ball bearings or elements thereof may be investigated by a simulation method so that valuable data may be obtained through measurements of torque and electrical resistance, the measurements providing an indication of operating efficiency. Since the apparatus is a simulation apparatus, interchangeability of simulated bearing components is permitted to some extent. The device provides output data respective to the torque transmitted through the ball elements as well as the electrical resistance to currents passing through the lubricating film associated with those ball elements. Changes in efficiency of the simulated bearing are generally indicated by rising or erratic torque or electrical resistance indications that can be used to predict failure well in advance of the failure event.
However, like many of the prior art test concepts, studies using the Howe apparatus are performed on simulated bearing arrangements and the concept does not readily permit the direct testing of commercially produced bearings or the testing of lubricants directly in such commercial bearings. In addition, it is not possible to test lubricants or bearings on which rotor assemblies, such as gyroscope rotors, are mounted in the form of production assemblies. Such tests are clearly desired, since some of the most influential and erratic sources of undesirable loading in ball bearings are those resulting during assembly of a bearing into the instrument in which it is to be used, such as in a gyroscope rotor or precision motor assembly. In general, the nature of such prior test devices precludes studies of bearings in actual operating conditions; they are basically research tools, while the present invention may be used either as a research tool or as a quality control monitor in design and production.
The reliability and life of a ball bearing is a function of its lubricating film thickness with respect to minute irregularities in the surface finish of the bearing balls and races. The film thickness of consequence is that of the plateau at the central contact locus of the lubricating film. The trailing edge of this separating film as a bearing ball passes a given location on a race is reduced in thickness by the hydraulic, elastic, hydrodynamic, and other forces involved. It has been determined experimentally that bearing life is most closely associated with the lubrication film thickness in the central plateau region and is not significantly related to that at the exit region restriction which is normally measured by electrical resistance measurements. Thus, electrical resistance measurements, while useful for many purposes, are often not as accurate an indicator of bearing life as is desired. Furthermore, it is not possible to search a bearing race for local imperfections, such as for areas in which a sufficient lubrication film does not exist due to an oleophobic surface, misalignments, or local raceway surface defects.
It is recognized that ultrasonic energy has been employed in the past for various testing purposes, such as, for example, the detection of hidden flaws deep within solid structures. In such conventional ultrasound test equipment, for example, a pulse of energy is directed through a medium toward a defect or other interface and the time of arrival of the reflected wave at a receiver is measured in order to afford a measurement of the depth of the flaw or other interface. However, the technique lends itself to use only in situations in which the flaw or interface to be observed is buried relatively deeply within the medium. Thus, the direct measurement of films as thin as bearing lubrication films would not prove to be practical using this conventional technique. Accordingly, it is believed that the prior art does not include satisfactory ultrasonic methods for the investigation of rotating or stationary ball bearing assemblies and of the lubricant films usually employed within them.